The solid oxide fuel/electrolyzer cell (SOFC) is one of the most sought-after technologies for converting chemical energy into electrical energy and producing hydrogen. The chemical energy is supplied by reacting fuel (such as hydrogen) and oxidants (such as air). When consuming pure hydrogen, SOFCs produces no greenhouse gasses. SOFCs can be run in reverse, as an electrolyzer cell, using electricity and steam and producing hydrogen. The produced hydrogen can then be used as an energy source elsewhere.
A SOFC has an anode, a cathode, and a solid electrolyte. The operating temperature of a SOFC is rather high, generally ranging from 600° C. to 900° C. Therefore, ceramic materials are used in SOFC as the solid electrolyte, cathode, and anode. As a single SOFC does not produce enough energy or hydrogen (when run in reverse), they are generally stacked together in a SOFC stack. SOFC stacks require interconnects to transfer fuel, air, and electricity through each cell in the stack. Particularly, the fuel channels and the air channels have to be well sealed from each other to maximize efficiency.
A seal in a SOFC stack ideally must prevent the mixing of fuel and oxidant, prevent the mixing of the fuel and oxidant with the ambient environment, provide mechanical bonding of the components, and provide electrical insulation between the components of the stack.
Fuel cells increase in efficiency as operating temperatures increase. However, at high operating temperatures, the integrity of the cells degrades. Degradation in the SOFC stack causes the fuel, oxidants and other reactant gases to leak and mix, which reduces efficiency and long term stability of the stack.
Glass and glass-ceramic seals are a common seal used in fuel cells. Although inexpensive, current glass and glass-ceramic seals are brittle, and over time degrade. For example, most seals do not have a suitable coefficient of thermal expansion (CTE), resulting in stress fractures in the glass as well as additional stress on other components in the SOFC stack. Current glass and glass-ceramic seals also are known to volatize during operation (e.g. silica, borate, and other alkali metals), which may foul or poison the electrodes and interact in an undesirable manner with other SOFC components.
U.S. Pat. No. 5,453,311 discloses a SrO-alumina-borosilicate sealing glass containing a high amount of B2O3 and La2O3 with a coefficient of thermal expansion (CTE) of 8.5-12*10−6/° C. in 50-600° C. and glass transition temperature of 500-750° C. However, these glasses are not thermally stable at high operating temperatures, such as 800° C. Since the glass contains a high amount of B2O3, evaporation of boron oxides may take place, degrading the cells performance as the B2O3 evaporates, causing the seal to fail.
U.S. Pat. No. 6,291,092 discloses a B2O free, alkali oxide and BaO sealing glass. However, the presence of alkali oxide decreases the electrical resistance due to the mobility of monovalent cations in the glass. The monovalent cations of the alkali oxides can easily diffuse to the other sealed components in the SOFC Stack, which changes the glass compositions and thus degrades the cell's long term stability. Furthermore, the presence of BaO in the glass reduces both thermal and chemical stability. BaO-containing glasses crystallize near 750° C. to different polymorphs of barium-alumina-silicate phase with different coefficients of thermal expansion (CTE), leading to thermal stress. Moreover, BaO reacts severely with chromium containing metallic interconnects to form BaCrO4, which has about twice the coefficient of thermal expansion (CTE) of the sealing glass and induces cracks in the seal.
Metal brazes, which use a molten metal filler to ensure sealing, are another form of seal used in a SOFC stack. Although easy to manufacture, the brazes are electrically conductive, making them unsuitable for most seals. Few braze metals are compatible with the SOFC operating conditions, and commonly are made of expensive noble metals. There have been efforts to use silver, but its use in both oxidizing and reducing environments can lead to chemical instability.
Therefore, there exists a need for a seal capable of operating in a SOFC stack while preventing the mixing of fuel and oxidant, preventing the mixing of the fuel and oxidant with the ambient environment, providing mechanical bonding of the components, and providing electrical insulation between the components of the stack.